I don't think you will ever solve the problem with the current way of maintaining the BBS layer. As long as everyone can freely add to it, it is bound to end up as a useless mass of redundant placemarks. New people don't know about the rules, and moderators can inform about it thousands of time, it's not going to help. That is the price you have to pay when making things open for everyone to maintain, it seems like a good idea at the start, but it usually ends up as something that runs out of control. Just wait until people begin spamming the BBS layer with all forms of advertising as they do in emails.

Another thing: It is not always that simple to just say "turn on the BBS layer" or "do a search" before posting a placemark to see if it is already there, because some of the additions include many placemarks that is somehow connected, and it would be wrong to leave one out just because someone else has already posted it. Let's say that you want to post a folder containing placemarks of rollercosters in various amusement parks (it does actually exist). Should you then leave out the ones that are already mentioned, so you don't get double placemarks? I think that would be wrong. The collection should contain all rollercoasters regardless of other peoples existing placemarks.